Machining metal parts

etpm

Gold Member
Joined
Jun 30, 2021
Messages
295
Location
Whidbey Island, WA
Tractor
yanmar ym2310
More about scales for a DRO. Magnetic and capacitive scales are analog while the glass scales are digital. This means that the position of the readhead is done differently depending on the scale. The magnetic and capacitive scales have a signal that changes gradually as the readhead passes by. Because of the way the signal changes the resolution can theoretically be very fine, much finer than what the display shows. But because the signal is also subject to electrical noise, which must be filtered, the practical resolution is much lower than theoretical resolution. Capacitive scales are so noisy they are limited to a practical resolution of .0005. Magnetic scales are way less noisy so the practical resolution can be much finer, even ten times finer. When I say the scale is noisy what I really mean is that the way the scale reading system works is noisy. Also, because of this electrical noise capacitive scales tend to not be as repeatable as other scale types. Magnetic and capacitive scales require some pretty complex electronics to be read, part of the reason for the noise in the electronics. On the other hand, glass scales are truly digital. They have discrete lines on the scales that are read by passing light through the scale. The light is either blocked or not. So the signal is either on or off. This means the scales are simple to read. It also means they have a certain fineness of resolution that cannot be made finer. They are typically read in quadrature because they have two LEDs and two photo receptors to do the reading. The LED and photo receptor pairs are staggered so that as the readhead is moved pair A senses the leading edge of a line on the scale, then pair B senses the leading edge, then pair A senses the trailing edge, then pair B senses the trailing edge. So, 4 simple on or off signals per line. This divides the spacing of the lines by 4, meaning that the scale can be read to a resolution 4 times finer than the marks on the scale. So even though the scale resolution does not change a finer resolution can be had by how the scale is read. The accuracy of the scale depends on the accuracy of the lines on the scale. These days it is possible to make very accurately placed lines on the scales for very little cost. Because the position of the lines on the scale cannot change, because no interpolation is done, glass scales repeat. You can always count on the readout showing the same position when the readhead is moved a certain amount. Even if the scale is inaccurate the inaccuracy will always be the same. Because the output from a glass scale is purely digital, just on or off, the signal is easily used for other things, like adding another display in another location so that you can have two readouts at once. Or the signal from the scale can be used to position a stepper motor.
Eric
 

jthibodeau89

Gold Member
Joined
Apr 28, 2017
Messages
370
Location
Shelbyville
Tractor
JD 4300, Agco Allis 5670
Etpm is correct that glass scales are more accurate. CMM machines use glass scales, but the are also typically used in a clean environment. If you can keep the scales clean, and get the exact length required for your application, then they are a gpod option as well.

It is intetesting to see how the glass scales are produced, if you are interested. Very vibration sensitive during the laser engraving.
 
Last edited:

rScotty

Elite Member
Joined
Apr 21, 2001
Messages
4,930
Location
Rural mountains - Colorado
Tractor
Kubota M59, JD530, JD310SG. Restoring Yanmar YM165D
Looks like my little Rockwell 1hp has a 3 axis X,Y,Z movement of 24"x6.5"x2.5".
In mm that is roughly 610x165x64 mm.
Since the table and quill are very accurate, I'd prefer resolution to 0.0002" ...but could live with .0005.
I doubt if I get .001 setting repeatability with the original hand dials.
rScotty
 

GeneV

Veteran Member
Joined
Aug 19, 2019
Messages
1,907
Location
Lake County, IL
Tractor
Bradley 48" stander MF GC1710
Looks like my little Rockwell 1hp has a 3 axis X,Y,Z movement of 24"x6.5"x2.5".
In mm that is roughly 610x165x64 mm.
Since the table and quill are very accurate, I'd prefer resolution to 0.0002" ...but could live with .0005.
I doubt if I get .001 setting repeatability with the original hand dials.
rScotty
It only has a 2.5" quill travel? In any case, I'd keep it simple if I was you, get x and y axis glass.

For the quill (z axis), just get a caliper type. This is the one I have mounted on my precision matthews: 6" Vertical Digital Machine Scale
 

rScotty

Elite Member
Joined
Apr 21, 2001
Messages
4,930
Location
Rural mountains - Colorado
Tractor
Kubota M59, JD530, JD310SG. Restoring Yanmar YM165D
No, it doesn't have much quill travel. I end up raising and lowering the table when I need more travel. Surprisingly, it will hold a couple of tenths (0.0002") when doing that.
I like the caliper idea.
thanks.
 

jwmorris

Platinum Member
Joined
Oct 3, 2007
Messages
743
Be sure to get magnetic scales, not glass (optical). Mag scales can be cut to length and coolant doesnt affect readings. Get as fine a scale as you can afford.

I’d rather have an old Bridgeport with glass scales than a new made in China with magnetic scales. They don’t have all of the features of the current displays but if you come across a good mill that has them, there is no reason to fix what’s not broken.

Not many home/hobby guys run flood coolant anyway.
 
 
Top